Cadmium-evolving process.



WOOLSEY MUA. JOHNSON, OF HARTFORD, CONNECTICUT.

CADMIUM-EVOLVING IROCESS.

No Drawing.

To all whom it may concern:

Be it known that I, WOOLSEY MoA. J OHN- soN, a citizen of the United States, and residing at Hartford, in the county of Hartford and State of Connecticut, have invented a new and Improved Cadmium-Evolving Process,- of which the following specification is a full disclosure.

This invention proposes a treatment capable of deriving from cadmium-bearing materials a greater yield of that metal than has heretofore been commercially attainable and, from a morespecific aspect, this invention contemplates a reformation of present methods of deriving zinc from its ores so as to render available a'treatment of a differential character which, while effecting a more complete recovery of the cadmium will at the same time realize a more complete yield of the companion metals and values such as zinc, copper, lead and the precious metals.

At the present time, cadmium is commercially obtained as a by-product incidental to the recovery of zinc by the so-called retorting method now in current use, and a predominant characteristic of that method is that the liberation of the cadmium in the ultimately prepared charge ensues mainly coincidentally with that of the zinc. Consequently, the cadmium to a large extent finds its way into the blue-powder to form a small constituent element thereof, and this blue-powder is the ultimate source ofsuch cadmium as is now commercially available. Its isolation from the zinc is of necessity accomplished'through a rather intreatment of the cadmium-bearing bluepowder. Much of the evolved cadmium, however, fails to becarried into the bluepowder and accordingly is non-recoverable for the reason that it condenses with the zinc to form a permanent element of the spelter; sometimes in a proportion as high as 2% thereof. The cadmium thus confined by the spelter not only represents a loss directly proportional to the value of the cadmium, but it also impairs the'value of-the'spelter when present'inmaterial amounts because it diminishes the adaptabilityofthe spelter Specification of Letters Patent. Application filed July 30, 1914. Serial No. 854,169

elaborated treatment and re Patented July 2, 1918.

for certain purposes; notably for rolling into sheet zinc.

Now by this invention there is created a process of smelting zinc ores so formulated that, not only will higher yields of zinc and companion metals be obtainable, but the loss of cadmium will also be greatly decreased and its recovery free from zinc and in a form suitable for various purposes will be facilitated.

The primary characteristics of this method may be better revealed by assuming a typical ore-analysis, and then explaining one of the various Ways in which it wouldv be treated in accordance with this invention; and illustrating the various typical changes in composition resulting during the progressive stages of this treatment.

Assume a native zinc-cadmium sulfid ore analyzing about as follows:

It may be enplained that the 'ore repre Zn; 0.70% Cd; 2.076% Pb; CaO; 10.8%

sented by (A) is too high in sulfur for sub- 1 sequent treatment and it must be prepared by first being subjected to a special roasting or calcining operation. In contra-distinction from the conventional roasting of retorting practice, this preliminary calcining operation preferably does not eliminate all sulfur and it is so conducted as not to expose the ore for a material length of time to a temperature exceeding 800 C. (as distinguished from the usual 1000 C. conventionally employed) in order that the loss of cadmium in the fume from the roaster will be kept as small as possible.

It should here be explained that, under ordinary roasting conditions and at a temperature of 800 C., CdS is oxidizable to CdO at a sufficient rate or speed of reaction mium blue-powder) at a temperature of about 730 0., as contrasted with the temcadmium (as a gas which changes to cadperature of1040 (3., required for the initial reduction of ZnO to Zn.

1 have, however, discovered that in the presence of considerable amounts of the usual sulfur-bearing materials the roasting may be conducted to an extent satisfying the needs of the further steps of my process and at a sufficient speed by employing a temperature of about 800 C. In other words, instead of forcing the roasting to the point of a practically complete elimination of the sulfur, as is now done with present zinc-retorting 111ethods,1preferably propose so to modify the prevailing roasting step as to leave considerable amounts of sul fur in the ore, say about 5%, and thus avoid the stringent forcing conditions that have heretofore obtained, and which have caused very great losses of cadmium during the roasting operation. It should be eX- plained that cadmium oxid is to a small extent volatile in the sweep of the gases of a roasting furnace and, as this sweep is considerably higher under the higher temperature conditions of conventional roasting, the loss is correspondingly greater due to this cause than in my process in which this step is more moderate. Thus, as a result of my special temperature roast, the above mentioned typical ore would be caused to analyze about as follows:

(B) 31.2 Zn; 0.93 Cd; 2.3 Pb; 17.4%

Fe; 4.8% Cu, 4.0% CaO; 14.2% SiO' The above product (B) has, in a sense an' anomalous composition, inasmuch as it is unsuitable either for conventional retorting practice (by reason of its comparatively high content of sulfur) or for economical electric smelting, since its constitution prohibits it from bemg entirely and completely smelted into slag, matte, fused-bullion, gases and vaporized metal and it, therefore, demands to be modified in composition to a greater or less degree so that its immediate reduction may be accomplished continuously and without undue consumption of current. In this instance, but little modification is demanded, since the above roasted product (B) happens to be about completely selffluxing since it needs the addition of only about 4% of CaO, in the form of quick lime, together with about 22% of soft-coal (analyzin 75% fixed carbon, 20% volatile matter an 5% ash) the theoretical carbon requirements being 16.3% for the reduction to sulfids of the CdO, Fe O CuO and any sulfates. In certain' instances, the product at the end of the roasting operation will be of a more pronounced non-fiuxing character; thus rendering necessary a more radical reconstruction of its composition by providing it with the necessary amount of carbon and slagging constitutents. In any event, a charge analyzlng about as follows is to be obtained from the roasted product:

(C) 24.0 Zn; .71 Cd; 1.8 Pb; 13.4 Fe; 3.7 Cu; 6.14 CaO; 11.3 $0,; 4.6 S; 12.7 fixed carbon.

This charge (C) is then subjected to a preliminary heat treatment devised to efi'ect a substantially complete recovery of the cadmium without appreciably evolving zinc, and at the same time be adapted to effect a metallurgical alteration (as by reducing Fe 0 (Inf) and sulfates) in the charge of such a character as will particularly adapt it for an economical electric-furnace treatment. Suitable amounts of iron, silica, or old slag, lime or the like may be added to the product (B) so that after the preliminary cadmium-evolving heat-treatment the resultant product will be constituted to smelt with a production of enough slag to keep the furnace open and free working.

(D) 25.7% Zn; 0% Cd; 1.8% Pb; 14.3% Fe; 3.96% Cu; 6.6% @210; 13.1% SiO 4.93% S; fixed carbon.

This cadmiumevolving heattreatment was carried out under reducing conditions by causing the charge C (which is B, modified as stated) to pass through a suitably constructed furnace such as a mufile-furnace of the Hasencleoer type so as to pre-heat and pre-reduce the mix.

This intermediate or cadmium evolving furnace is constructed to permitthe temperature to be controlled so as to prevent it from rising above 1000 C. for any preventable periods and for enabling it to be maintained above 800 C. The charge, which preferably contains soft coal or other carbonaceous material of such character as to evolve reducing gas at the temperature of the furnace, is caused to feed into the furnace and to be therein subjected to the tem perature thereof for such a time as will be adequate to effect an evolution of the cadmium in the form of cadmium blue-powder. Preferably the reduction 'will be effected in the presence of a stream of suitable reducing gas, such as will be evolved-by the soft coal or other volatile carbonaceous material preferably used in the charge, so as to sweep the cadmium blue-powder into a suitable condenser. By conducting this operation under the conditions specified, a reaction will ensue and the cadmium will be isolated from the zinc (which remains unreduced and unvolatilized and confined to the charge) and a further important constitutional alteration will take place in the charge. That is to say, the iron oxids, in the presence of the carbon and at the temthe electric-furnace.

The cadmium blue-powder (product E) evolyed analyzing 75% Cd during the 1 execution of this step, will be collected in prolongs suitably arranged and further refined in any suitable manner, such as that conventionally practised, but it is noteworthy that this blue-powder is (in contradistinction from that derived as a result of the conventional retorting practice) characteristically free from substantial amounts of zinc. It is also noteworthy that the amount of cadmium recovered at this point greatly exceeds that recoverable by the retorting method, inasmuch as the loss during the roasting operation is reduced to a minimum and all the cadmium is extracted at a point in advance of the evolution of the zinc so that it is prevented from entering either the spelter or the zinc-blue-powder.

The typicalore that which was utilized for the purpose of revealing the more fundamental characteristics of this invention is of course to be understood as exemplifying a variety of ores. Certain modifications and refinements are contemplated in the case of special ores. For example when the ore contains large P158 or PbSO, which would be reduced to PbS (under pre-heating con: ditions) the volatile emanations would be passed through an apparatus for precipitating the metallic values, as for instance by passing the emanations through settling chambers or through a small bag house or through .a Cottrell electrostatic apparatus so as to obtain a precipitate or condensate containing considerable PbS and PbSO, (the latter caused by oxidation of PbS) as well as metallic cadmium. This will better be understood by explaining that lead-sulfid (PbS) has an appreciable vapor tension at 800 degrees C. to 900 degrees C. so that the resultant vapor existing in the pre-heating furnace will be swept away in the current of gas along with the cadmium vapor and,

T when there is much PbS in the mixture (say from 10% to 15%) the precipitated cadmium blue-powder will be contaminated by the lead. A consequential saving of the lead that otherwise would be lost is thus effected because this lead would otherwise have mixed with and deteriorated the spelter and never be isolated therefrom, whereas when concentrated in the cadmium bluepowder, it will be recovered during the subsequent isolation of the cadmium.

Under such conditions it is proposed to subject the contaminated cadmium bluepowded to a 10% solution of sulfuric acid so as to dissolve the cadmium sulfate and relieve an insoluble residue lead sulfate which may be isolated by a filter press and reduced during the electric smelting operations. The cadmium may be recovered from the filtrate in any suitable manner, as by electrolyzing it in a nearly neutral state in vats, using a lead-anode and forming cathodes of pure cadmium which can then be melted 1nto commercial form. As the cadmium is-present in a finely divided form in the condensate it will readily react with the diluted acid, so the control of the acidity of the solution is almost automatic.

The de-cadmiumized residue (product D; may now be treated in any appropriate manner to recover its metallic values. It may be explained, however, that the foregoing treatment was devised not only to isolate the cadmium values but at the same time to condition the residue for a continuous electric smelting of the zinc in accordance with my U. S. Patent #868345. On the other hand, if circumstances are such that it would be more profitable to produce a non-slagging mixture with a corresponding conditioning of the charge, the de-cadmiumized residue could be treated by charging into an ordinary retorting furnace.

In the case instanced, charge D would be passed into a suitably constructed electric furnace and smelted electrically to obtain cadmium free spelter and cadmium free blue-powder.

The spelter thus made by electric smelting of this de-cadmiumized product shows onanalysis less than 0.01% Cd and is preferable for many purposes.

Having thus revealed this invention, I claim as new the following combination of steps, or of equivalents thereof, and desire to secure the same by Letters Patent of the United States 1. A process of the nature disclosedfor deriving cadmium from sulfur-bearing ores which consists in preliminarily effecting an incomplete roasting of the ore to reduce the sulfur contained therein to not materially less than about 3%, and thereafter subjectingthepartially-roasted material to reducing conditions to reduce and evolve cadmium contained therein.

2. A process of the nature disclosed for the deriving of cadmium from a sulfur-bearing material which consists in preliminarily efiecting an incomplete roasting of the material and subsequently subjecting the incompletely-roasted material to reducing conditions in the presence of a current of gas effective to carry off the evolved cadmium vapors.

3. A process of the nature disclosed for differentially reducing a cadmium-zinc bearing material; comprising obtaining the material with a considerable sulfur-content and then subjecting the said sulfur-bearing material under reducing conditions to a temperature sufiiciently high to evolve cadmium vapors but too low to evolve zinc vapors.

4:. A process of the nature disclosed for treating a sulfur-bearing zinc-cadmium product which comprises subjecting it to a prolonged treatment under reducing conditions at a temperature below 1000 C. to evolve cadmium blue-powder without effecting a substantial reduction of the zinc, and then subjecting. the residue while containing a material amount of sulfur to a temperature exceeding 1000 C. under reducing conditions to evolve condensable zinc vapors.

- 5. A process of the nature disclosed for differentially deriving cadmium and zinc from a sulfur-bearing product containing the same, which comprises the step of preliminarily and incompletely desulfurizing the product and then subjecting the partially -desulfurized product for a prolonged period to a current of gas under reducing conditions until the greater proportion of the cadium present has been reduced and vaporized.

6. A process of the nature disclosed for continuously producing cadmium from material containing not less than about 3% of sulfur consisting in subjecting the sulfurbearing material under reducing conditions to a .temperature sufliciently high to effect an evolution of cadmium blueowder and too low to effect an evolution 0 the zinc.

7. A process of the nature disclosed for deriving cadmium from sulfur-bearing ores which consists in preliminarily efi'ecting an incomplete roasting of the ore at a moderate temperature to eliminate the greater amount of sulfur contained therein, then discontinuing the roasting before the sulfur content is reduced sufficiently to permit the ore to be retorted; and then sub ecting the partially-roasted material to reducing conditions to liberate the cadmium contained therein. 8. A process of the nature disclosed for the deriving of cadmium from a sulfur-bearing material which comprises the preliminary step of incompletely eliminating sulfur from said material and the subsequent step of subjecting the material while containing considerable amounts of sulfur to reducing conditions in the presence of a current of gas effective to carry off the evolved cadmium vapors. I

9. A process of the nature disclosed for differentially obtaining cadmiuin and zinc from sulfur-bearing material contained in the same, consisting in first largely but not wholly reducing the sulfur-content of the material; then subjecting the incompletely desulfurized material under reducing conditions to a temperature effective to evolve cadmium vapors but ineffective to reduce zinc vapors.

10. A process of the nature disclosed for treating unsmelted ores containing Cd and Zn which comprises first reducing CdO to Cd at a temperature below the reduction temperature of zinc oxid; evolving same in a gaseous condition; collecting same in a condenser; then immediately treating the decadmiumized residue at a materially higher temperature to make a spelter containing a trace only of Cd.

11. A' process for conditioning ores to yield cadmium-free zinc which consists in, preparing a mixture of ore and solid carbon having the capacity of innately generating non-oxidizing gas, by mixing with carbonaceous material undistilled ore analyzing at least 30% zinc and less than 1% cadmium, and then propelling said mixture through a heated air-excluded zone at a temperature less than that required to reduce zinc-oxid and suflicient to reduce all of the cadmium- 12. A process for preparing cadmium-free zinc-gas which consists in first roasting the ore tooxidized metallic sulfids, then causing the ores in the presence of a reducing agent to pass through a zone heated to a temperature not exceeding 1000 C. and not less than 800 C. whereby all particles of said ore will be exposed to a temperature not exceeding 1000 0. whereby all of the cadmium may be expelled as a metallic vapor; and then subjecting said preheated ore to zinc reducing conditions at a temperature above 1000 C. to evolve zinc-gas.

13. Aprocess for preparing cadmium-free spelter which consists in first roasting the ore to oxidized metallic sulfids, then causing the ore in the presence of a reducing agent to pass through a zone heated to a temperature not exceeding 1000 C. and not less than 800 0. whereby all particles of said ore will be exposed to a temperaturenot exceeding 1000 C. whereby all of the cadmium may be expelled as a metallic vapor; and then subjecting said preheated ore to zinc reducing conditions.

14. A process for conditioning ores to yield a cadmium-free zinc which consists in preparing a mixture of ore and solid carbon having the capacity of innately generating non-oxidizing gas, by mixing with carbonaceous material undistilled ore analyzing at least 30% zinc and less than 1% cadmium, and then propelling said mixture through a heated air-excluded zone at a temperature less than duce zinc-oxid and suflicient to reduce all of the cadmium-oxid.

that required to re 15. A process .for obtaining cadmium In witness whereof, Lhereunto subscribe from spelter or from ores bearing zinc and my name, as attested by the two subscrib- 10 cadmifim which consists in first roasting the ing witnesses. oret en causing t e ore to ass throu h a 5 zone heated to reducing conditions at a tim WOOLSEY JOHNSON perature below 1000 to expel the cadmium Witnesses: as a metallic vapor and then subjecting the DELOS G. HAYNES, product to zinc reducing conditions. B. COOKE. 

